1. Field of the Invention
This invention relates to electrostatic separation process and apparatus for the removal of acetonitrile from C.sub.4 and C.sub.5 hydrocarbon streams by passing a stream of water in counterflow to the direction of flow of the hydrocarbon through an electric field of varying electric field gradient established by a plurality of composite electrodes.
2. Description of the Prior Art
The Clean Air Act Amendments of 1990 have forced refiners to search for ways to introduce oxygen into gasoline to produce cleaner burning reformulated fuels. The leading component to satisfy these needs is Methyl Tertiary Butyl Ether (MTBE). MTBE has a high blending octane number and relatively low vapor pressure and is an excellent blending component. Other ethers presently to enter this market are tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE).
MTBE is formed by the reaction of isobutylene and methanol at mild operating conditions (100.degree.-180.degree. F., 100 psig) over a catalyst. The high selectivity of the reaction at these conditions allows 94-95% of the reactive hydrocarbon to be converted to MTBE as limited by equilibrium constraints. By using a catalytic distillation column, essentially complete conversion is attainable. TAME and ETBE are formed in comparable processes by the reaction of isoamylene with methanol and the reaction of isobutylene with methanol, respectively.
The etherification processes utilize strongly acidic ion exchange resins as etherification catalysts. These are strongly acidic organic polymers. As an isobutylene or isoamylene molecule meets alcohol at an active site, the reaction takes place rapidly forming ether.
The activity of the catalyst for the etherification reaction is a function of the acid loading or capacity of the resin. This functionality is not linear; a loss of 20% of acid sites on the catalyst gives approximately 50% loss of activity for conversion to MTBE. It is therefore important to minimize the deactivation of the catalyst with effective feed pretreatment to maintain peak performance and long catalyst life.
The loss of catalytic activity may be caused by the adsorption of basic compounds or metal ions, the blockage of the active sites by polymeric products, or by the splitting off the functional groups due to long term operation at temperatures above 240.degree. F. The latter two causes are affected by the operating conditions of the MTBE unit. The major source of lost activity is typically from poisons entering with the feedstocks to the unit. Poisons to the catalyst include basic compounds such as ammonia, amines, caustic soda, and acetonitrile (ACN).
In refinery applications, the largest source of hydrocarbon feedstock containing isobutylene is the C.sub.4 stream from the cat cracking unit (FCCU). Some C.sub.4 's are also obtained from fluid or delayed cokers. ACN is formed in these units that enters the etherification process with the hydrocarbon feed stream. The amount of ACN in the feed varies with the severity of the cat cracker operation, crude source, and catalyst used in the FCCU. The ACN level of refinery based MTBE unit feeds may range from &lt;10 ppm to &gt;550 ppm. Unlike all the other feed poisons which deactivate the catalyst in a plug flow fashion through the catalyst bed, ACN's deactivation mechanism is not immediate and results in a diffused deactivation throughout the entire bed. Catalyst deactivation by ACN occurs through the catalyzed hydrolysis of ACN to acetic acid and ammonia and the subsequent neutralization of the acid sites by the ammonia.
In order to obtain adequate run lengths with the catalyst and optimum performance, the first step in the etherification process is a feed pretreatment step designed to remove the poisons to very low levels (&lt;1 ppm). Since the poisons are much more soluble in water than hydrocarbon, the common treatment is a multistage water wash. The water and hydrocarbon streams are contacted utilizing trays or packing. In the tower the continuous water phase flows down the column as the liquid hydrocarbon droplets are dispersed upwards. Of the many poisons to the catalyst, ACN is the most troublesome. The tower design is based on ACN removal to 1 ppm. The design variables include the number of theoretical contact stages and the flow rate of water. In most typical refinery MTBE units, a minimum of three contacting stages and at relative flow rate of 30 weight percent water to hydrocarbon is required to reduce ACN levels to the 1 ppm specification. This results in a tower containing at least three beds of at least 8 feet of packing in each bed, or 12-16 trays. The column must also contain sufficient height to allow the less dense hydrocarbon phase to separate from the water phase. This is important as free water can have an adverse effect on the catalyst.
The amount of wash water required is also an important design variable. Wash water flow at 20 weight percent of the hydrocarbon is a minimum amount based on the efficiency of the liquid-liquid contacting. In many cases, much higher rates are used. This in turn results in a large flow of waste water extract leaving the column which must be handled either by reusing it in other refinery processes or more commonly, discharging it to the effluent treating plant.
In summary, an important part of any refinery based etherification process is feed pretreatment to remove catalyst poisons so that economical catalyst life and high ether production rates are achieved. Early MTBE plants have water wash systems designed before the importance of ACN removal was recognized. Inadequate removal of ACN in those units gave catalyst bed life as short as six months. Water wash systems designed to effectively remove ACN has demonstrated catalyst life from 12 to 24 months. Optimization of this step to make it more efficient resulting in reduced capital investment, operating expense, and water usage is extremely attractive.
A liquid-liquid extraction process has three steps:
1. Intimate contact between the two phases PA1 2. Coalescence of dispersed phase drops PA1 3. Separation of the phases
Conventional liquid-liquid extraction devices use mechanical energy to create drops. The rate of mass transfer is proportional to the interfacial area, so one strives to create dispersed phase drops as small as practical. If the drop size is too small, residence time required for phase separation makes the contactor too large and too costly. Conventional phase contact devices generally use minimum dispersed phase drop diameters of approximately 0.5-1.0 millimeter.
Extraction processes are often used when distillation is difficult or ineffective. Extraction utilizes differences in the solubilities of the components rather than differences in their volatilites. Extraction takes advantages of chemical differences between components rather than vapor pressure differences as in distillation.
In liquid-liquid extraction two phases must be brought into good contact to permit transfer of material and then be separated. In extraction, since the two phases have comparable densities, the energy available for mixing and separation is small. The two phases are often hard to mix and harder to separate. The viscosities of both phases, also, are relatively high, and linear velocities through most extraction equipment are low. Therefore, in some types of extractors, energy for mixing and separation is supplied mechanically. This requires additional expense in equipment, maintenance, and operating costs.
U.S. Pat. No. 4,702,815 discloses a system for removing brine from oil well production. A fresh water or less saline water is passed in counterflow to the oil well production through electric fields established by composite electrodes.
U.S. Pat. No. 4,804,553 discloses a countercurrent dilution water flow system coupled with the electrostatic mixing of the dilution water with the brine inherent in oil well production. A plurality of parallel conductive electrode plates in which the voltage applied to the electrode plates is modulated becoming the equivalent of a multi-stage mixer/coalescer/separator.
U.S. Pat. No. 4,606,801 discloses a method and apparatus for dispersing or mixing relatively polar fluids in a relatively non-polar fluid. The fluids are passed between electrostatic fields that are modulated to effectively mix and separate these fluids.
The present invention is an improvement over conventional extraction techniques. It is an advantage that conventional type mixing and separation equipment are not needed. Generally, the electrostatic separation systems have been applied to the removal of connate insolubles in oil streams and other solid/liquid dispersions, no procedure has addressed the removal of hydrocarbon soluble materials into water by liquid-liquid extraction.